In recent years, an increase in the level of UV rays reaching the surface of the earth, caused by partial depletion of the ozone layer, is regarded problematic, and there is demand for sun-screening cosmetics having a higher effect than conventional. Conventionally, organic compounds, for example those based on benzophenone, benzoic acid or methoxycinnamic acid, are used as UV absorbers in such sun-screening cosmetics, but these organic compounds cannot absorb UV rays in a broader wavelength range and irritate the skin. Because of these problems, there is a need for safer UV-screening agents.
It has been revealed that UV rays reaching the surface of the earth cause skin aging and cancers by the synergistic effect of UV rays of short wavelengths (UV-B region: 280 to 320 nm) causing inflammations (so-called sunburns) such as blisters and red spots, with UV rays of longer wavelengths (UV-A region: 320 to 400 nm), and in recent years, there is significant interest in screening of UV rays particularly in the UV-A region.
Zinc oxide originally shows a sharp edge of absorption in the vicinity of 380 nm, and thus has a higher screening effect against UV rays in the UV-A region, and later, superfine particles of zinc oxide have been developed. The superfine particles of zinc oxide shield from UV rays in a broad wavelength range from the UV-B region to the UV-A region, and the superfine particles of zinc oxide are excellent in transparency with a lower refractive index of 2.0 than the refractive index (2.7) of superfine particles of rutile titanium oxide, and thus attract attention as a UV-shielding agent.
On one hand, zinc oxide is inherently soluble in a very small amount in water, and by virtue of the physiological action of its eluted zinc ion, it has been utilized for a long time as an astringent in the field of cosmetics. Further, zinc oxide has such chemical reactivity as to react with fatty acid to form metallic soap, thus absorbing sebum secreted from the skin to improve retention of cosmetics on the skin and absorbing body-odor components to exhibit a deodorant effect.
However, such physiological activity and chemical reactivity tend to be further increased by converting zinc oxide into superfine particles, and when higher safety for skin tissues is to be required in the future, the activity of the particle surface is desirably suppressed.
The water solubility and chemical reactivity of zinc oxide described above bring about various inconveniences in designing the formulation of cosmetics. That is, the most important starting component in cosmetics is water also serving as a major constituent component in the human body, and in the cosmetics containing superfine particle of zinc oxide, its zinc ion eluted into water reacts with another component, and thus the ratio of water cannot be increased, thus limiting the freedom of formulation. In the case of sun-screening cosmetics containing conventional superfine particles of zinc oxide, for example, it is difficult to increase the ratio of water to 50% or more, and the majority of emulsified products are limited to those of water-in-oil type (W/O). These emulsified products are based on oil, thus hardly achieving dry feeling.
Use of the superfine particles of zinc oxide in cosmetics also limits the freedom of formation because the superfine particles react with other various ingredients such as an oil, perfume, coloring matter, organic UV absorber and water-soluble polymer in the cosmetics, to cause problems such as an increase or decrease in the viscosity of the cosmetics, generation of a nasty smell, discoloration etc.
Under these circumstances, JP-A 3-183620 discloses an art wherein the surfaces of fine particles of zinc oxide are coated with an oxide or hydroxide of aluminum, silicon, zirconium or tin thereby reducing the catalytic activity of zinc oxide to suppress the denaturation of organic vehicles in cosmetics and further reducing the dynamic coefficient of friction of zinc oxide to prevent deterioration in smoothness and feeling.
JP-A 11-302015 discloses an art which contains arranging a high-density coating layer consisting of silicon oxide on the surface of a zinc oxide particle to suppress the solubility thereof in purified water.
JP-A 8-53568 discloses an art which includes converting a monomer phase having zinc oxide dispersed therein into particles by suspension polymerization or emulsion polymerization to suppress the action of zinc oxide on other components such as vehicles. Generally, the monomer used in the suspension or emulsion system includes styrene, methyl methacrylate, acrylonitrile, vinyl acetate etc., but any of the resulting polymers are poor in chemical resistance.
Methyl methacrylate used in the Examples in JP-A 8-53568 supra has an ester linkage, and thus it cannot be said that the resulting polymer is satisfactory in respect of chemical resistance. Further, the resulting particles are generally swollen with ethanol or oil incorporated into cosmetics, thus failing to arrive at substantially sufficient inhibition of the reaction of zinc oxide with other components such as vehicles.
On one hand, fine particles themselves generally easily form an aggregate. Accordingly, fine particles are dispersed in a monomer and fixed by polymerizing the monomer, or fine particles are kneaded with, and dispersed in, a molten resin and then fixed by solidifying the resin.
In the method of fixing fine particles in resin by solidifying the resin, the resin is sprayed in a molten state into a gaseous phase in order to reduce the particle diameter of the resin, or the resin is solidified and then milled.
Accordingly, JP-A 6-167835 discloses a technique of spray-cooling a molten resin via a rotating disc type nozzle.